1. Field of the Invention
The present invention relates to an injection-molding machine for injection-molding a thermosetting material, and to a method of working such a machine.
More particularly, the invention relates to vulcanization of rubber. However, it can also be used in similar technical fields, such as, in particular, injection-molding or shaping silicones, phenolic resins, or indeed polyurethane resins.
2. Description of the Related Art
Conventionally, a rubber vulcanization machine firstly includes means for plasticizing and injecting the thermosetting material. The state of the art provides various alternatives, as regards the structure of such means.
Thus, a first solution uses separate plasticizing means that include a conveyor screw that heats the material. Such plasticizing means are associated with distinct injection means that comprise, in particular, a piston associated with a chamber for receiving the material that has been made malleable.
By way of a variant, the plasticizing means can be combined with the injection means. In which case, injection is of the “screw-piston” type, in which the plasticizing screw also acts as an injection piston.
Finally, another alternative is of the First-In-First-Out (FIFO) type. In that alternative, the plasticizing takes place though the injection piston.
After the initial plasticizing and injection stage, implemented using one of the above-described possibilities, the malleable material is injected into a flow channel whose downstream end defines at least one outlet orifice. Said outlet orifice opens out into feed channels for feeding a mold, which channels are themselves in communication with the cavities of the mold.
Thus, in the flow channel, which is situated upstream from the mold, the malleable material substantially does not set in service, due to the operating conditions such as, in particular, temperature and flow speed. However, in the bottom volume of the mold, namely in the feed channels and in the cavities, the malleable material does set, due to the temperature and to the stagnation to which it is subjected. Indeed, the outlet orifice of the flow channel has a small cross-section, making it possible to separate the fraction of the material that has set from the fraction of said material that has not, after such setting has taken place.
A common problem when manufacturing products made of rubber lies in the vulcanization time, it being desirable to reduce this time to as short as possible. That problem is broached, for example, in U.S. Pat. No. 6,280,175.
That document teaches providing a constriction in the flow channel, i.e. in a zone in which the malleable material does not set, the cross-section of that constriction not being circular, but rather being generally elongate in the manner of an ellipse. In that way, the rubber is heated uniformly, at all points of its cross-section. In other words, the presence of that constriction makes it possible to bring a substantial quantity of heat to the central stream of rubber, thereby making it possible to reduce the corresponding vulcanization time.
Unfortunately, the solution described in that U.S. Pat. No. 6,280,175 suffers from other drawbacks. It is accompanied by heating that can be excessive and generally non-uniform and that can locally degrade the material, and therefore give rise to defects in the finished piece.
Alternative solutions are also known, in which devices are used to modify the direction of flow of the malleable material. However, unlike in the teaching of the above-mentioned US patent, those devices are placed in the mold proper, i.e. in a zone in which the malleable material sets.
More precisely, EP-A-1 186 339 describes a static mixer designed to homogenize the malleable material. The use of that device gives rise to an increase in the temperature of each fluid stream, thereby giving rise to problems analogous to those encountered when implementing the above-described US patent.
In addition, U.S. Pat. No. 4,199,315 discloses a pneumatic tire injection-molding machine in which, inside the mold, the malleable material is firstly separated into two branch streams. Upstream from the branching point, a cold central stream and a warmer peripheral stream are to be found. Then, within each branch flow, a cold stream and a hot stream are to be found that flow substantially symmetrically about the longitudinal axis of each branch channel.
That document then teaches processing each branch flow by changing the flow paths of the respective hot and cold streams. That solution cannot apply to a flow that is not subdivided, in which the cold central stream is surrounded by a peripheral hot stream. In addition, it is advantageous in very specific uses only, such as injection molding of pneumatic tires.